Heat and water vapor exchangers (also sometimes referred to as humidifiers) have been developed for a variety of applications, including building ventilation (HVAC), medical and respiratory applications, gas drying, and more recently for the humidification of fuel cell reactants for electrical power generation. Many such devices involve the use of a water-permeable membrane via which water vapor and, provided there is a temperature differential across the membrane, heat is transferred between fluid streams flowing on opposite sides of the membrane.
Planar plate-type heat and water vapor exchangers use membrane plates that are constructed using discrete pieces of a planar, water-permeable membrane (for example, Nafion ®, cellulose, polymers or other synthetic or natural membranes) supported by a separator material and/or frame. The membrane plates are typically stacked, sealed and configured to accommodate fluid streams flowing in either cross-flow or counter-flow configurations between alternate plate pairs, so that heat and water vapor is transferred via the membrane, while limiting the cross-over or cross-contamination of the fluid streams.
A heat recovery ventilator (HRV) is a mechanical device that incorporates a heat exchanger with a ventilation system for providing controlled ventilation into a building. The heat exchanger heats or cools the incoming fresh air using the exhaust air. Devices that exchange moisture in addition to heat between the two air streams are generally referred to as Energy Recovery Ventilators (ERVs), sometimes also referred to as Enthalpy Recovery Ventilators. Two primary reasons to install an ERV are increased energy savings and improved indoor air quality. ERV systems typically comprise a sheet metal enclosure, fans to move the air streams, ducting, as well as filters, control electronics and other components. The key component in the ERV which transfers the heat and water vapor between the air streams is called the ERV core. Often ERV cores are constructed like the planar plate-type heat and water vapor exchangers described above.
A benefit of planar plate-type heat and water vapor exchanger designs for ERV and other applications, is that they are readily scalable. The quantity as well as the dimensions of the modular membrane plates can be adjusted for different end-use applications. However, with plate-type planar exchangers there are a large number of joints and edges that need to be sealed. As a result, such devices can be labor intensive and expensive to manufacture. Also their durability can be limited, with potential delamination of the membrane from the frame and failure of the seals resulting in leaks, poor performance and cross-over contamination (leakage between streams).
In other heat and water vapor exchanger designs, the many separate membrane plates are replaced by a single membrane cartridge made by folding a continuous strip of membrane in a concertina, zig-zig or accordion-fashion, with a series of parallel alternating folds. Similarly, for heat exchangers, a continuous strip of material can be patterned with fold lines and folded along such lines to arrive at a configuration appropriate for heat exchange. By folding the membrane in this way, the number of edges that must be bonded can be greatly reduced. For example, instead of having to bond two edges per layers, it may be necessary only to bond one edge per layer because the other edge is a folded edge. However, the flow configurations that are achievable with concertina-style pleated membrane cartridges are limited, and there is still typically a need for some edge sealing. Another disadvantage is the higher pressure drop as a result of the often smaller size of the entrance and exit areas to the pleated cartridge.